A standard box-cutting knife such as described in PCT publication WO 92/16337 of P. Hodgson (British priority 22 Mar. 1991) has a housing, a blade holder slidable in the housing, and a blade in the holder displaceable with the holder between a front position projecting from the front end of the housing and a rear position recessed therein. A spring is provided that continuously urges the blade and holder into the rear position and a thumb button is provided that allows the user to push out the blade and hold it out so the knife can be used. When the thumb button is released the blade retracts back into the housing automatically. Such a knife is quite safe, as the blade will retract automatically except when the thumb button is depressed, but is inconvenient as the user must maintain pressure on the thumb button to use the knife.
U.S. Pat. No. 3,999,290 of Wood describes a system where instead of a thumb button a drag element is connected to the holder and projects forward out of the housing. The outer end of this drag element is positioned so that if the front end of the knife is pressed against, for instance, a box to be cut and the knife is pulled backward, the drag element will catch on the box and pull the blade and its holder into the front position, allowing the blade to cut the box. While such a cutter is effective on rough cardboard cartons, the drag element can slip on smoother materials so the blade will not be extended automatically. When the material being cut is very soft, the drag element tears it, making a sloppy cut. It is also possible for the drag element to disengage from the material, allowing the blade to retract so that the user must restart the cut.
Another system is described in German patent 3,744,456 of Rehm. It has a wheel on the front forward edge of the knife that is attached through a gear linkage with the blade holder so that as the wheel is rotated the blade is extended, and when the wheel is maintained in the rotated position the blade stays extended. Such a system only works when the wheel is pressed with quite some lateral force against the object to be cut, which is not always desirable in particular when the object is soft. Furthermore the gear structure is fairly complex so it adds quite a bit to the cost of the tool.
German utility model 1,885,827 describes a double-bladed utility knife with fixedly mounted blades at opposite ends of a handle/housing. Respective guards can be retracted into the handle/housing against the force of a common biasing spring to expose the blades and can be hooked to stay in the retracted position. Such an arrangement is not automatic at all and, even so, is fairly complex.